Gene therapy by direct delivery to the respiratory tract remains the principal strategy in the development of new treatment for human respiratory diseases such as cystic fibrosis. This effort has been hampered by lack of safe and effective methods to deliver this recombinant material. We have previously shown uptake and expression in the respiratory epithelium of rhesus monkeys treated by aerosol with cationic liposomes complexed with CFTR cDNA, although these results had been somewhat variable. Improvements in liposome formulation may have overcome these problems. We evaluated the immediate safety and efficacy of an improved liposome formulation in the delivery of cftr to the lower airway. Animals were sedated and allowed to breath spontaneously the dna-liposome complex using a unique low pressure closed system. No immediate adverse effects were evident, and neither animal exhibited any signs of respiratory distress. Three days after treatment the lungs were evaluated for immunohistochemical evidence of cftr overexpression. Both animals demonstrated cftr expression throughout the conducting airways, with much greater staining evident in the animal treated with the higher dose. We conclude that liposomes are an effective method of transferring exogenous genes to the lower respiratory tract when administered by aerosol.